Electronic calipers are becoming increasingly common in manufacturing industries. Capacitive position transducers are almost universally used as the principle component of electronic calipers for measuring thickness and other physical parameters. These capacitive transducers include a pair of substrates that move relative to each other along a measurement axis. A set of electrodes is carried by each of the substrates, with the electrodes of one substrate being positioned close to the electrodes on the other substrate to form a capacitor therebetween. The electrodes on each of the substrates are connected to conventional circuitry for providing an indication of the relative position between the two substrates as a function of the capacitance between various electrodes in each set. The capacitive position transducer may be of the incremental type, in which the circuitry provides only an indication of incremental movement from a known point, or an absolute position type, in which the circuitry provides an indication of the relative position between the two substrates regardless of whether their initial relative position is known. Incremental and absolute position type position transducers are disclosed in U.S. Pat. No. 4,420,754 and 4,879,508.
Although these calipers are sometimes used in dry, relatively clean, environments, such as inspection rooms or engineering offices, such calipers are often used to measure dimensions of work pieces in machine shops and in other relatively dirty environments. When used in these environments, such calipers can become contaminated by particulate matter and fluids, such as cooling or cutting fluids. The liquid or particulate contaminants find their way between each set of electrodes and affect the capacitance between the electrodes in a manner that is not related to the relative position between the substrates along the measurement axis. Contaminants between the electrodes of a capacitive position transducer degrade performance in two respects. First, the particulate or fluid may be a dielectric having a dielectric constant that is different from the dielectric constant of air that is in an air gap between the sets of electrodes. The capacitance between electrodes on opposite sides of the contaminate will thus be greater than the capacitance between other electrodes having the same relative geometry which do not have contaminants therebetween. Under these circumstances, the capacitance between the electrodes will not provide an accurate indication of the relative position between the substrates.
The second effect of contaminants between electrodes results when the contaminants are resistive. A resistive substance between the electrodes will cause the signal coupled between the electrodes to be differentiated with respect to time. The time constant of the differentiation is a function of both the conductivity of the contaminant and the capacitance between the electrodes. Where the time constant is relatively short, the amplitude of the signal may decay so rapidly that it cannot be sensed by conventional circuitry for capacitive position transducers.
One approach to minimizing the adverse effects of contaminants in capacitive position transducers is described in U.S. Pat. No. 5,172,485 to Gerhard et al. Gerhard et al. teach coating the electrodes of each substrate with a thin layer of dielectric material and then mounting the substrate so that the dielectric material coating the electrodes of one substrate slides along the dielectric material coating the electrodes of the other substrate. In theory, the sliding contact between the dielectric layers eliminates any air gap for contaminants to fill. However, the sliding contact approach described in the Gerhard et al. patent requires that the substrates be resiliently biased toward each other so that deviations from exact surface flatness and alignment can be accommodated by allowing the substrates to move apart. However, the compliant nature of the suspension for the substrates allows the substrates to be forced apart from each other by particulate contaminants which inevitably collect between the electrodes when the capacitive position transducer is used in a dirty environment. Thus, the approach of reducing the gap between electrodes and, in the extreme case, eliminating the gap altogether, has not proven to be adequate under some circumstances.
One approach to isolating a variety of precision devices from liquid and particulate contaminants is to seal sensitive components of the devices from the external environment. However, it has not been thought possible to effectively seal the space between the electrodes of capacitive position transducers used in calipers because the slide containing one set of electrodes must slide across the exposed surface of the scale containing the other set of electrodes. As a result, the space between the two sets of electrodes is inherently exposed to the external environment.